This invention relates to reagents for the desulphurization of iron melts such as pig iron and cast iron and the use of the reagent for said purpose.
The desulfurization of molten iron, outside a blast furnace, in the open ladle or in the torpedo is well known to those skilled in the art. Calcium carbide or lime-based mixtures have been used as reagents for many years and have been found to be efficient with respect to causing rapid and significant removal of the sulfur from the iron.
Most recently, specific reagents have been disclosed in U.S. Pat. Nos. 4,764,211 and 4,832,739 as containing from about 50-98 percent, by weight, of calcium carbide and 2-50 percent, same basis, of a dried coal which contains at least 15 percent, by weight, of volatile components and which evolves a gas volume of at least 80 standard liters of gas/kg of coal. This prior art product is described as not introducing further slag-forming components into the iron melt, evolving an adequate amount of gas for the dispersion of the calcium carbide, possessing advantageous consumption values, causing short treatment times and resulting in high sulfur removal.
SUMMARY OF THE INVENTION
A novel reagent for the desulfurization of molten iron has been discovered which is based on calcium carbide and/or lime and which contains, as a gas generating component, an asphaltite. The reagent is chemically engineered to maximize the desulfurizing efficiency of all its components. Since the asphaltite is available as a fine powder, it may be mixed with the other component(s) without milling. Its use is therefore less of a safety hazard than a milled mixture of calcium carbide and/or lime and coal which, due to the temperatures generated during milling, may spontaneously combust when exposed to air.
The use of an asphaltite for gas generation is advantageous over volatile coals in that it contains considerably less oxygen. The corresponding decrease in the oxygen available upon volitalization substantially increases the reagent's desulphurization efficiency. The higher percentages of hydrogen and free carbon in the asphaltites also provide an enhanced environment in the gas generated plume for deoxidization of the hot metal. Additionally, the asphaltites typically contain less sulphur and fixed carbon than volatile coals. These differences act to increase the reagent's desulphurizing efficiency by minimizing sulphur input and to decrease slag production by minimizing the fixed carbon remaining in the kish. The higher percentage of volatiles in the asphaltites (approximately 85 percent) relate to lower addition levels required for equivalent mixing as provided by the volatile coals (approximately 40 percent). This naturally leads to the production of lower slag quantities as increased levels of desulfurizing components can then be utilized to decrease the overall quantities of reagent required for equivalent desulphurization.
As is mentioned in previous patents, it is considered advantageous to have the volatile contents of the gas generator released as quickly as possible upon contact with the molten bath. Gilsonite in particular, has been shown in explosion tests to have a maximum rate of pressure rise of 3,700 psi per second as compared to 2,300 psi per second for 37 percent volatile coal. This attribute is considered advantageous in improving the distribution of the desulphurizer immediately upon immersion into the molten iron.